The present invention relates to optical communications, and more particularly, to optical communications using a tunable fiber optic filter.
Transmission of information by light over optical fibers is widely used in telecommunication systems. Optical signals are generated, transported along optical fibers and detected to regenerate the original electronic signal with as little change as possible. Optical fibers are combined with electronic signal processing to obtain a lower cost and higher quality digital transmission.
Fiber optic system applications may use direct optical processing of signals without conversion to electronic signals. This typically requires optical signal processors such as amplifiers, multiplex/demultiplexes, splitters, couplers, filters, equalizers, and switches adapted for use with optical fibers. Optical fiber filters are important components for such fiber optic systems. Many different types of optical filters for selecting light of a particular wavelength have been proposed in conjunction with wavelength division multiplexed optical systems for use in optical communication systems.
A fiber Fabry-Perot (FFP) interferometric filter is such a filter. The Fabry-Perot FP Interferometer is described by C. Fabry and A. Perot in 1897 (Ann. Chem. Phys., 12:459-501) and is widely used in a variety of applications of optical filters. The basic structure and operation of the FP interferometer is well-known in the art and is described in many physics and optics texts. This interferometer includes an optical cavity formed between two typically highly reflecting, low-loss, partially transmitting mirrors. Lenses are typically used to collimate divergent optical beams for processing through the FP interferometer.
Fabry-Perot filters include fixed-wavelength FFPs and tunable-wavelength FFPs. Changing the distance between optic fiber ends in the cavity or stretching an optical fiber in the cavity tunes the wavelength. Tuning can be accomplished by controlled temperature variation of the FFP (temperature-tuned) or by changing the relative position of the ferrule elements, without destroying alignment, for example, by electromechanical actuators such as piezoelectric transducers. The response of such filters typically changes with changes in environmental conditions such as temperature, pressure or aging of the filter. Variable voltage and thermal fluctuations (drifts), as well as the non-linearity of the electromechanical actuators and FFP optics, may require constant calibration of the tunable filter when used as an optical channel analyzer, for example.
Controllers for the filters adjust the filter to minimize the signal loss at the selected laser wavelength, while attenuating those lasers which have different wavelengths. An example is the controller disclosed in U.S. Pat. No. 5,838,437 to Miller et al. Conventional controllers use analog RF techniques which have large power requirements for a battery operated remote controller. Typical controllers, for example, may have limited feature sets and use about 6 Watts. Battery life is short and manual control is required to operate the filter. Thus, there is a need for a digital controller which supports an advanced feature set and operates at much lower power to extend battery life.
In view of the foregoing background, it is therefore an object of the invention to provide a digital controller, for an optical system, which operates at relatively low power to extend battery life.
This and other objects, features and advantages in accordance with the present invention are provided by a fiber optic system including an optical fiber carrying at least one optical signal having a wavelength peak, an optical detector coupled to the optical fiber for detecting a current optical level (e.g. a photon power level), and a tunable optical filter coupled to the optical fiber upstream from the optical detector. The system preferably further includes a controller connected to the optical detector and the tunable optical filter, for stepping the tunable optical filter over a sequence of wavelengths while analyzing respective optical levels. The controller preferably reverses the stepping direction of the tunable optical filter, based upon the current optical level being less than a prior optical level, to locate the wavelength peak of the optical signal.
The controller may include a sample comparison circuit for comparing the current optical level with the prior optical level. The sample comparison circuit preferably reduces a sample rate when the wavelength peak of the at least one optical signal is located, to thereby reduce a power consumption of the controller. The sample comparison circuit may include a first comparator for comparing the current optical level with a threshold, and a second comparator for comparing the current optical level with the prior optical level. The controller does not reverse the stepping direction of the tunable optical filter if the current optical level is less than the threshold.
The optical detector may generate an analog optical level signal representing the current optical level, and the sample comparison circuit may include an analog-to-digital (A/D) converter for converting the analog optical level signal into a digital optical level signal for use by the first and second comparators. The controller may also include a bus controller for generating a location information signal based upon an output from the sample comparison circuit, and a digital-to-analog (D/A) conversion circuit for converting the location information signal into a control voltage for the tunable optical filter. The D/A conversion circuit may include a range and offset selection circuit for selecting a range and offset for the sequence of wavelengths.
Objects, features and advantages in accordance with the present invention are also provided by a method for controlling a fiber optic tunable filter, including detecting a current optical level of an optical signal in an optical fiber, stepping the tunable optical filter over a sequence of wavelengths while analyzing respective optical levels, and reversing the stepping direction of the tunable optical filter, when the current optical level is less than a prior optical level. This permits locating the wavelength peak of the optical signal. The method may include comparing the current optical level with the prior optical level, and comparing the current optical level with a threshold. Again, the stepping direction of the tunable optical filter would not be reversed if the current optical level is less than the threshold.
The current optical level may be represented by an analog optical level signal, and the method may include converting the analog optical level signal into a digital optical level signal for the comparing. Also, the method may include generating a digital location information signal based upon the comparison of the current optical level with the prior optical level, and converting the digital location information signal into a control voltage for the tunable optical filter. Furthermore, a sample rate is preferably reduced when the wavelength peak of the optical signal is located, to thereby reduce power consumption.
The system, controller and method preferbaly involve the conversion of an analog input signal into a digital signal for processing, and the generation of an analog control voltage to the tunable filter. The digital processing, including a reduced sample rate, may reduce power consumption by about 150/1 over conventional analog systems.